1. Field of the Invention
In the art of tempering shaped glass sheets, it is common to support a glass sheet on an outline or ring type mold of relatively rigid metal that comprises an outline upper surface shaped to conform to the shape desired for the glass and to sag the glass sheet into conformity with the outline shaping surface by heating the glass laden mold until the glass is sufficiently soft to sag and conform to the shape provided by the shaping surface. When the glass reaches the shaping surface, it is quenched suddenly and rapidly by the application of air blasts against the supported lower surface and against the upper exposed surface of the glass sheet.
If the glass sheet contacts the hot heavy metal rail during this rapid cooling, glass breakage can result from surface vents at the glass-metal contact areas. In the past, glass sheet tempering molds of the outline type have been reduced in thickness along their glass engaging upper edge to reduce the effective heat transferred at the glass engaging surface of the mold or the molds have been provided with ribbons of insulating material such as asbestos or heat resisting materials in the form of a strip of fiber glass to space the supported glass sheet from direct contact with the heavy metal of the outline mold.
U.S. Pat. No. 2,297,315 to Owen discloses such an arrangement. Since it is necessary to replace the ribbon periodically, such a technique is useful only in an operation where a series of molds are used for conveying glass sheets through a bending furnace and then through a quenching station, so that extra molds may be kept in reserve to enable operators to replace worn out ribbon that requires replacement and have an inventory of outline molds ready to replace any production molds whose ribbons are worn out without shutting down the operation to await the replacement of a worn out ribbon or strip on the outline mold.
In apparatus for bending an annealing glass sheets, where the change in temperature is not as sudden as in the temperature cycles common to bending and tempering operations, the difference in thermal response to a change in temperature between the relatively heavy, bending mold having high heat capacity per unit area and the glass sheet having low heat capacity per unit area can be overcome to a certain extent by providing relatively small metal shaping rails that contact the supported glass and that are adjustably secured to relatively rigid stiffening rails. U.S. Pat. No. 3,088,300 to Elliott discloses such an arrangement.
The use of even small metal members or members with serrated edges to contact the glass in spaced relation to the relatively rigid stiffening members is not suitable for use in tempering glass because rapid cooling of the glass is accompanied by a slower rate of cooling by the heated metal portion of the mold in contact therewith and the difference in cooling between the hot glass and the hot metal provides the glass sheet in contact with the metal with a retarded cooling rate compared to the glass not in contact with the metal. This difference in cooling rate between the glass sheet in contact with the metal of the mold shaping surface and the portion of the glass that is completely exposed to the air blasts has resulted in surface tension stresses in the glass. Under certain conditions, the surface tension stresses can be sufficient to cause breakage.
With the advent of the air form method of making shaped tempered glass sheets, wherein the glass sheets are shaped to conform with a mold that holds the glass thereagainst by vacuum and then deposits it on a relatively cool or unheated outline ring-type mold, such as disclosed in U.S. Pat. No. 3,846,106 to Seymour, the outline glass supporting ring that supports the shaped glass sheet during quenching is provided with a reduced thickness and a notched and/or apertured construction in the edge portion that engages the shaped glass to minimize the portion of the relatively cool metal that makes contact with the hotter glass sheet. However, the residual glass to metal contact remaining even with such construction was still sometimes sufficient to cause some breakage. The glass tempering art still needed to develop a technique for further reducing glass breakage resulting from the establishment of too high a thermal gradient from the portion of the supported glass sheet engaged by the shaped sheet supporting outline support structure to a portion of the supported glass sheet adjacent thereto.
The advent of the air form operation which requires the use of a single mold for supporting each glass sheet in succession during the rapid cooling or quenching step makes it impossible to use the winding strand or strip of asbestos or other heat-resisting material such as fiber glass without requiring the entire line to be shut down whenever the strip wears out and requires replacement.
There still remains the need for the glass tempering art to discover a material that is both sufficiently rugged to avoid the need for frequent replacement and is less likely to cause glass sheet breakage during tempering than a metal mold, particularly when glass sheets are shaped and tempered using the air form type of operation.